1. Field of the Invention
This invention relates to an improved method of controlling cyanides in the water system of an FCC reactor.
2. Description of the Prior Art
Refinery cyanides cause considerable corrosion, hydrogen blistering and waste water treatment problems. Majority of refinery cyanides are formed in the reactor of fluid catalytic cracking (FCC) units. In a typical refinery, a portion of the nitrogen from the crude enters in the feed to the catalytic cracker, wherein a large amount of the nitrogen is liberated as ammonia and a small amount as cyanide. All of the gas from the FCC reactor, including the ammonia and the cyanide, is carried overhead into the distillation column, where water is injected into the overhead stream for controlling corrosion problems. The resulting sour water is separated from the hydrocarbon products at various stages in the system, usually downstream of the FCC reactor. Most of the cyanides in the sour water become complexed with metal and are therefore known as complex cyanides. These complex cyanides are very difficult to remove from the water. With increased dependence of the refineries on sour crude containing progressively higher amounts of nitrogen, larger throughputs, and more severe processing conditions dictated by processing changes, corrosion and hydrogen blistering in sour water refinery overhead systems have recently increased dramatically.
In addition to cyanide-containing water from the FCC unit, significant concentrations of complex cyanides are also found in the waste water from coker units of the refinery. However, cyanides are produced in the coker units to a lesser extent than in the cracking units. This is due to the absence of a cracking catalyst, lower temperature and therefore lower severity of cracking. Coker effluent also contains a relatively large amount of water, originally added as steam in product strippers, coke drums and furnace coils. Accordingly, the concentration of cyanides in the water stream exiting the coker is relatively low.
In a typical refinery all of the cyanide-bearing streams are conducted to a sour water system wherein simple cyanides are stripped from the water in conventional stripping columns. However, complex cyanides (e.g., complexes of the cyanides with iron or other metals) are not readily strippable in such conventional stripping columns.
One of the previously known methods of controlling cyanide concentration in sour water streams involved injecting sulfur-containing compounds, i.e., polysulfides, such as sodium and ammonium polysulfides (APS), into the sour water system. The polysulfides convert the cyanide into non-corrosive, biodegradable thiocyanate before the complex non-strippable cyanide/metal complexes are formed. Thiocyanates are water soluble and they can be readily removed from the sour water by stripping. Prior art attempts of controlling cyanide content of sour water systems by the APS injection were often unsuccessful because it was found that a certain amount of cyanides could not be eliminated by injection and steam stripping. More particularly, the prior art relied heavily on steam stripping of the cyanides in conventional sour water strippers and on the treatment of the sour water stripper bottoms with sulphur. It was thought that some of the cyanides could be removed by steam stripping from the sour water system, and the remainder could be complexed with elemental sulphur which was used to contact sour water stripper bottoms. However, it was found that these attempts were largely unsuccessful because, it is believed, the cyanides in the sour water stripper bottoms were mostly complex cyanides. Complex cyanides, it is believed, are formed in the overhead portion of the FCC main fractionating column and in the unsaturated gas plant, usually located downstream of the FCC main fractionating column. Complex cyanides cannot be readily removed in steam strippers. Accordingly, some refineries experienced considerable problems with polysulfide injection into FCC units. Such problems involve, e.g., severe fouling and plugging of the sour water strippers (e.g., see Kunz et al, "Refinery Cyanides-A Regulatory Dilemma", Hydrocarbon Processing, October 1978, pages 98-106).